1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the invention relates to an image forming apparatus, suitable for a printer head using LED's (light-emitting diodes) or an LCD (liquid-crystal display), a dot array printer or the like, in which, by using a high-quality lens array redundant in alignment in a sub-scanning direction so as to hardly generate nonuniformity in the amount of light to influence the picture quality even if alignment between an LED array and the lens array deviates in the sub-scanning direction by a predetermined amount, an high-quality image can be obtained although the adjustment of the alignment is easy or unnecessary.
2. Description of the Related Art
FIG. 1 is a schematic diagram illustrating a principal portion of a method for measuring and controlling nonuniformity in the amount of light in a lens array in an image forming apparatus using the lens array.
In FIG. 1, light-source means 51 comprises an LED array in which a plurality of LED's are arranged in a one-dimensional direction. A lens array (imaging means) 52 is provided by arranging a plurality of condensing lenses (rod lenses) in two lines in a scanning direction in a close-packed state. The close-packed state is a state in which lenses in one line are staggered and closely placed on lenses in another line. This lens array 52 is also named a "two-line lens array". Measuring means 53 comprises, for example, a photosensor. Output means 54 comprises, for example, a display, and displays an output signal (representing the amount of light) obtained by the photosensor 53.
In FIG. 1, the plurality of LED's constituting the LED array 51 are all lit. The amount of emission (emission pattern) of light beams from the plurality of LED's is sensed by performing scanning by the photosensor 53 via the two-line lens array 52. An output signal obtained at that time from the photosensor 53 is displayed on the display 54. Nonuniformity in terms of the amount of amplitude (nonuniformity in the amount of light) is obtained from the maximum value (MAX) and the minimum value (MIN) of the displayed data. Thus, nonuniformity is confirmed and controlled.
It is known, however, that tolerance of human visual characteristics for nonuniformity depends not only on the above-described amount of amplitude but also on a spatial frequency. That is, as shown in FIGS. 2(A) and 2(B), tolerance of human visual characteristics for nonuniformity differs depending on the spatial frequency for the same amount of amplitude (MAX-MIN).
The state of nonuniformity in the spatial frequency greatly changes if the alignment between the LED array 51 and the two-line lens array 52 in the sub-scanning direction (in directions indicated by a two-headed arrow A) deviates, so that the picture quality greatly changes depending on the adjustment of the alignment in the sub-scanning direction. Hence, conventionally, there is the problem that it is necessary to very precisely adjust the alignment of the lens array in order to suppress the generation of nonuniformity.